1. Field of the Invention
The present invention relates to a spindle motor for a hard disc drive (hereinafter referred to as “HDD”) having a structure to protect a magnetic head incorporated in the HDD from electrostatic discharge.
2. Description of the Related Art
Recently, a computer product incorporating an HDD becomes smaller in size and larger in capacity. In this sense, a spindle motor for an HDD also becomes smaller in size and operates in high precision and reliability.
FIG. 7 is a cross sectional view of a spindle motor for HDD. In FIG. 7, a spindle motor 50C for an HDD is basically composed of a stator 103 having a motor base 101, a shaft 90 standing on the motor base 101, a rotor hub 108 rotationally attached to the shaft 90, a stator core 102 having a plurality of prominence poles winded with a driving coil 105 and fixed on the motor base 101.
The rotor hub 108 is fixed with a cylindrical shaped rotor yoke 106 on the bottom portion. A cylindrical ring magnet 107 is fixed in the inner circumference of the rotor yoke 106. FIG. 8 is an enlarged cross sectional view of the rotor hub incorporating a pair of ball bearing shown in FIG. 7. In FIG. 8, the rotor hub 108 has an upper circumference portion 155 and a lower circumference portion 157. Each upper and lower circumference portion has adhesive-grooves 153a and 153b respectively. The upper circumference portion 155 has ball bearing 4A pressed and fixed therein. The lower circumference portion 157 has ball bearing 4B pressed and fixed therein.
The rotor hub 108 also has a cylindrical portion 151 and a flange 152 for fixing a position of a magnetic recording disc 10. The cylindrical ring magnet 107 is placed in a position opposed to the outer circumference of the plurality of prominence poles (not shown) of the stator core 102 in predetermined distance.
The pair of ball bearings 4A and 4B makes smooth rotation of the rotor hub 108 and placed between the shaft 9 and the rotor hub 108. The ball bearing 4A is incorporated with a stainless steel ball 41 and the ball bearing 4B is incorporated with a stainless steel ball 42.
As shown in FIG. 7, the motor base 101 has a circular protrusion 111, which holds the shaft 109.
The spindle motor 50C is also composed of a cap 110, which prevents oil and dust inside the motor from leaking out on the surface of the magnetic recording disc 10. The rotation of the rotor 108 is controlled by switching power supply to the driving coil 105 wounded in the stator core 102.
In FIG. 8, the ball bearings 4A and 4B are pressed into the upper circumference portion 155 and lower circumference portion 157 applied with adhesive. The ball bearing 4A has an outer portion 43b and the ball bearing 4B has an outer portion 44b. The outer portions 43b and 44b fit respectively with step portions 154 and 156 formed in the inner side of the rotor hub 108. The ball bearings 4A and 4B fit inside the rotor hub 108 by the adhesive.
FIG. 9 is a cross sectional view of the rotor hub incorporating a pair of ball bearings and a shaft. In FIG. 9, the shaft 9 having grooves 91 and 92 is applied with adhesive. Then the shaft 9 is stabilized by a stand 19, and is inserted in the center of the rotor hub 108 by a jig 18 with force F towards the direction of an arrow shown in FIG. 9. When the adhesive become solid in the grooves 91 and 92, and in the outer portions 43b and 44b, the shaft 9 is rotationally fixed with the rotor hub 108. The adhesive is also cured in inner portions 43a and 44a in opposite side of the outer portions 43b and 44b. 
After the adhesive is cured, the shaft 9 is taken out from the stand 19. Then the adhesive is applied to the lower portion 93 of the shaft 9. The lower portion 93 is inserted into the circular protrusion 111 formed in the motor base 101.
As mentioned above, a recent HDD is required to have high recording density and capacity. In order to provide such an HDD, the revolution of the HDD should be higher and a recording/reproducing head (not shown) should be small as possible. As for the recording/reproducing head of HDD, an MR (Magneto-Resistive) head or a G-MR (Giant-Magneto-Resistive) head is substituted for an MIG (Metal in Gap) head to comply with the high capacity HDD. However, such the MR head or G-MR head is easily damaged by electrostatic discharge.
In addition, the gap between the head and the magnetic recording disc became dozens of nanometers to a few nanometers, which provides even more large electrostatic charges easily. For example, the conductivity of the flange 152 and outer portion 43b are kept below a few ohms because they are pressed by the shaft 9. The conductivity of the outer portions 43b and 44b, and the inner portions 43a and 44a are kept under a few ohms when the motor remains still, and become a few mega ohms when the rotates.
As the ball bearings 4A and 4B, and the shaft 9 are fixed with keeping clearance of 3 to 7 μm by the adhesive filled in the clearance, the ball bearings 4A and 4B and the shaft 9 could not have sufficient conductivity unless they directly contact with each other.
The rotor hub 108 is made of Aluminum material, the outer portions 43b and 44b of the ball bearing, the stainless steel balls 41 and 42, and the inner portions 43a and 44a are made of conductive metal and contact with each other that static electricity may pass through. However, the inner portions 43a and 44a, and the shaft 9 are fixed with clearance of 3 to 7 μm and the adhesive is filled in the clearance, static electricity can not be discharged through the motor base 101.
Consequently, the charged voltage becomes higher so that the static electricity may pass through the head and destroy it.
In consideration of the above problem, the shaft 9 may be pressed into the inner portion 43a and 44a to directly contact with each other to discharge static electricity. However, this construction may affect the orbital plane of the ball bearing that NRRO (Non Repeatable Run Out) may be deteriorated.
Accordingly, the electrostatic discharge causes the destruction of the head and the related art described above can not solve this problem.